Point source transmission and speed-of-sound correction using multi-aperture ultrasound imaging

ABSTRACT

A Multiple Aperture Ultrasound Imaging system and methods of use are provided with any number of features. In some embodiments, a multi-aperture ultrasound imaging system is configured to transmit and receive ultrasound energy to and from separate physical ultrasound apertures. In some embodiments, a transmit aperture of a multi-aperture ultrasound imaging system is configured to transmit an omni-directional unfocused ultrasound waveform approximating a first point source through a target region. In some embodiments, the ultrasound energy is received with a single receiving aperture. In other embodiments, the ultrasound energy is received with multiple receiving apertures. Algorithms are described that can combine echoes received by one or more receiving apertures to form high resolution ultrasound images. Additional algorithms can solve for variations in tissue speed of sound, thus allowing the ultrasound system to be used virtually anywhere in or on the body.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/305,784, filed on Feb. 18, 2010, titled “AlternativeMethod for Medical Multi-Aperture Ultrasound Imaging”.

This application is also related to U.S. patent application Ser. No.11/865,501, filed Oct. 1, 2007, titled “Method and Apparatus to ProduceUltrasonic Images Using Multiple Apertures”, and to U.S. patentapplication Ser. No. 11/532,013, filed Sep. 14, 2006, titled “Method andApparatus to Visualize the Coronary Arteries Using Ultrasound”; all ofwhich are herein incorporated by reference in their entirety.

INCORPORATION BY REFERENCE

All publications, including patents and patent applications, mentionedin this specification are herein incorporated by reference in theirentirety to the same extent as if each individual publication wasspecifically and individually indicated to be incorporated by reference.

BACKGROUND OF THE INVENTION

In conventional ultrasonic imaging, a focused beam of ultrasound energyis transmitted into body tissues to be examined and the returned echoesare detected and plotted to form an image. The basic principles ofconventional ultrasonic imaging are well described in the first chapterof “Echocardiography,” by Harvey Feigenbaum (Lippincott Williams &Wilkins, 5th ed., Philadelphia, 1993).

In order to insonify body tissues, an ultrasound beam is typicallyformed and focused either by a phased array or a shaped transducer.Phased array ultrasound is a commonly used method of steering andfocusing a narrow ultrasound beam for forming images in medicalultrasonography. A phased array probe has many small ultrasonictransducer elements, each of which can be pulsed individually. Byvarying the timing of ultrasound pulses (e.g. by pulsing elements one byone in sequence along a row), a pattern of constructive interference isset up that results in a beam directed at a chosen angle. This is knownas beam steering. Such a steered ultrasound beam may then be sweptthrough the tissue or object being examined. Data from multiple beamsare then combined to make a visual image showing a slice through theobject.

Traditionally, the same transducer or array used for transmitting anultrasound beam is used to detect the returning echoes. This designconfiguration lies at the heart of one of the most significantlimitations in the use of ultrasonic imaging for medical purposes: poorlateral resolution. Theoretically, the lateral resolution could beimproved by increasing the width of the aperture of an ultrasonic probe,but practical problems involved with aperture size increase have keptapertures small. Unquestionably, ultrasonic imaging has been very usefuleven with this limitation, but it could be more effective with betterresolution.

In the practice of cardiology, for example, the limitation on singleaperture size is dictated by the space between the ribs (the intercostalspaces). Such intercostal apertures are typically limited to no morethan about one to two centimeters. For scanners intended for abdominaland other use, the limitation on aperture size is less a matter ofphysical constraints, and more a matter of difficulties in imageprocessing. The problem is that it is difficult to keep the elements ofa large aperture array in phase because the speed of ultrasoundtransmission varies with the type of tissue between the probe and thearea of interest. According to the book by Wells (cited above), thespeed varies up to plus or minus 10% within the soft tissues. When theaperture is kept small (e.g. less than about 2 cm), the interveningtissue is, to a first order of approximation, all the same and anyvariation is ignored. When the size of the aperture is increased toimprove the lateral resolution, the additional elements of a phasedarray may be out of phase and may actually degrade the image rather thanimproving it.

US Patent Application Publication 2008/0103393 to Specht teachesembodiments of ultrasound imaging systems utilizing multiple apertureswhich may be separated by greater distances, thereby producingsignificant improvements in lateral resolution of ultrasound images.

SUMMARY OF THE INVENTION

One embodiment of a method describes a method of constructing anultrasound image, comprising transmitting an omni-directional unfocusedultrasound waveform approximating a first point source within a transmitaperture on a first array through a target region, receiving ultrasoundechoes from the target region with first and second receiving elementsdisposed on a first receive aperture on a second array, the first arraybeing physically separated from the second array, determining a firsttime for the waveform to propagate from the first point source to afirst pixel location in the target region to the first receivingelement, and determining a second time for the waveform to propagatefrom the first point source to the first pixel location in the targetregion to the second receiving element, and forming a first ultrasoundimage of the first pixel by combining the echo received by the firstreceiving element at the first time with the echo received by the secondreceiving element at the second time.

In some embodiments, the method further comprises repeating thedetermining and forming steps for additional pixel locations in thetarget region. In one embodiment, additional pixel locations are locatedon a grid without scan-conversion.

In one embodiment, determining the first time and the second timecomprises assuming a uniform speed of sound.

In another embodiment, the method further comprises transmitting asecond omni-directional unfocused ultrasound waveform approximating asecond point source within the transmit aperture through the targetregion, receiving ultrasound echoes from the target region with firstand second receiving elements disposed on the first receive aperture,determining a third time for the second waveform to propagate from thesecond point source to the first pixel location in the target region tothe first receiving element, and determining a fourth time for thesecond waveform to propagate from the second point source to the firstpixel location in the target region to the second receiving element, andforming a second ultrasound image of the first pixel by combining theecho received by the first receiving element at the third time with theecho received by the second receiving element at the fourth time.

In some embodiments, the method further comprises combining the firstultrasound image with the second ultrasound image. The combining stepcan comprise coherent addition. In another embodiment, the combiningstep can comprise incoherent addition. In yet another embodiment, thecombining step can comprise a combination of coherent addition andincoherent addition.

In some embodiments, the method can further comprise receivingultrasound echoes from the target region with third and fourth receivingelements disposed on a second receive aperture on a third array, thethird array being physically separated from the first and second arrays,determining a third time for the waveform to propagate from the firstpoint source to the first pixel location in the target region to thethird receiving element, and determining a fourth time for the waveformto propagate from the first point source to the first pixel location inthe target region to the fourth receiving element, and forming a secondultrasound image of the first pixel by combining the echo received bythe third receiving element at the third time with the echo received bythe fourth receiving element at the fourth time.

In some embodiments, the method further comprises repeating thedetermining and forming steps for additional pixel locations in thetarget region. In some embodiments, the additional pixel locations arelocated on a grid without scan-conversion.

In one embodiment, the method further comprises transmitting a secondomni-directional unfocused ultrasound waveform approximating a secondpoint source within the transmit aperture through the target region,receiving ultrasound echoes from the target region with first and secondreceiving elements disposed on the first receive aperture and with thethird and fourth receiving elements disposed on the second receiveaperture, determining a fifth time for the second waveform to propagatefrom the second point source to the first pixel location in the targetregion to the first receiving element, determining a sixth time for thesecond waveform to propagate from the second point source to the firstpixel location in the target region to the second receiving element,determining a seventh time for the second waveform to propagate from thesecond point source to the first pixel location in the target region tothe third receiving element, determining an eighth time for the secondwaveform to propagate from the second point source to the first pixellocation in the target region to the fourth receiving element, andforming a third ultrasound image of the first pixel by combining theecho received by the first receiving element at the fifth time with theecho received by the second receiving element at the sixth time, andforming a fourth ultrasound image of the first pixel by combining theecho received by the third receiving element at the seventh time withthe echo received by the fourth receiving element at the eighth time.

In some embodiments, the method further comprises combining the first,second, third, and fourth ultrasound images. In some embodiments, thecombining step comprises coherent addition. In other embodiments, thecombining step comprises incoherent addition. In additional embodiments,the combining step comprises a combination of coherent addition andincoherent addition.

In some embodiments, the method comprises combining the first ultrasoundimage with the second ultrasound image. The combining step can comprisecoherent addition. In another embodiment, the combining step cancomprise incoherent addition. In yet another embodiment, the combiningstep can comprise a combination of coherent addition and incoherentaddition.

In some embodiments, the method further comprises comparing the firstultrasound image to the second, third, and fourth ultrasound images todetermine displacements of the second, third, and fourth ultrasoundimages relative to the first ultrasound image.

In another embodiment, the method further comprises correcting thedisplacements of the second, third, and fourth ultrasound imagesrelative to the first ultrasound image and then combining the first,second, third and fourth ultrasound images.

In an additional embodiment, the method comprises adjusting the third,fourth, fifth, sixth, seventh, and eighth times to correct thedisplacements of the second, third, and fourth ultrasound imagesrelative to the first ultrasound image.

In some embodiments, the method further comprises comparing the firstultrasound image to the second ultrasound image to determine adisplacement of the second ultrasound image relative to the firstultrasound image.

The method can further comprise correcting the displacement of thesecond ultrasound image relative to the first ultrasound image and thencombining the first and second ultrasound images.

In another embodiment, the method comprises adjusting the third time andthe fourth time to correct the displacement of the second ultrasoundimage relative to the first ultrasound image.

In some embodiments, the first pixel is disposed outside a plane definedby the point source, the first receiving element, and the secondreceiving element. In other embodiments, the first pixel is disposedinside a plane defined by the point source, the first receiving element,and the second receiving element.

Various embodiments of a multi-aperture ultrasound imaging system arealso provided, comprising a transmit aperture on a first arrayconfigured to transmit an omni-directional unfocused ultrasound waveformapproximating a first point source through a target region, a firstreceive aperture on a second array having first and second receivingelements, the second array being physically separated from the firstarray, wherein the first and second receiving elements are configured toreceive ultrasound echoes from the target region, and a control systemcoupled to the transmit aperture and the first receive aperture, thecontrol system configured to determine a first time for the waveform topropagate from the first point source to a first pixel location in thetarget region to the first receiving element, and is configured todetermine a second time for the waveform to propagate from the firstpoint source to the first pixel location in the target region to thesecond receiving element, the control system also being configured toform a first ultrasound image of the first pixel by combining the echoreceived by the first receiving element at the first time with the echoreceived by the second receiving element at the second time.

In some embodiments of the system, there are no transducer elementsdisposed between the physical separation of the transmit aperture andthe first receive aperture.

In one embodiment of the system, the transmit aperture and the firstreceive aperture are separated by at least twice a minimum wavelength oftransmission from the transmit aperture. In another embodiment, thetransmit aperture and the receive aperture comprise a total apertureranging from 2 cm to 10 cm.

in some embodiments, the ultrasound system further comprises a secondreceive aperture on a third array having third and fourth receivingelements, the third array being physically separated from the first andsecond arrays, wherein the third and fourth receiving elements areconfigured to receive ultrasound echoes from the target region.

In another embodiment of the multi-aperture ultrasound imaging system,the control system can be coupled to the transmit aperture and the firstand second receive apertures, wherein the control system is configuredto determine a third time for the waveform to propagate from the firstpoint source to a first pixel location in the target region to the thirdreceiving element, and is configured to determine a fourth time for thewaveform to propagate from the first point source to the first pixellocation in the target region to the fourth receiving element, thecontrol system also being configured to form a second ultrasound imageof the first pixel by combining the echo received by the third receivingelement at the third time with the echo received by the fourth receivingelement at the fourth time.

In some embodiments, the control system is configured to correct adisplacement of the second ultrasound image relative to the firstultrasound image due to speed of sound variation.

In other embodiments of the multi-aperture ultrasound imaging system,the transmit aperture, the first receive aperture, and the secondreceive aperture are not all in a single scan plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A. A two-aperture system.

FIG. 1B. Equidistant time delay points forming an ellipse around atransmit transducer element and receive transducer element.

FIG. 1C. Loci of points relative to equidistant time delays fordifferent receive transducer elements.

FIG. 2. A three-aperture system.

FIG. 3. Grid for display and coordinate system.

FIG. 4. Fat layer model with a three-aperture system.

FIG. 5. Construction for estimation of point spread function.

DETAILED DESCRIPTION OF THE INVENTION

Greatly improved lateral resolution in ultrasound imaging can beachieved by using multiple separate apertures for transmit and receivefunctions. Systems and methods herein may provide for both transmitfunctions from point sources and for compensation for variations in thespeed-of-sound of ultrasound pulses traveling through potentiallydiverse tissue types along a path between a transmit aperture and one ormore receive apertures. Such speed-of-sound compensation may beperformed by a combination of image comparison techniques (e.g.,cross-correlation), and the coherent and/or incoherent averaging of aplurality of received image frames.

As used herein the terms “ultrasound transducer” and “transducer” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies, and may refer without limitation toany single component capable of converting an electrical signal into anultrasonic signal and/or vice versa. For example, in some embodiments,an ultrasound transducer may comprise a piezoelectric device. In somealternative embodiments, ultrasound transducers may comprise capacitivemicromachined ultrasound transducers (CMUT). Transducers are oftenconfigured in arrays of multiple elements. An element of a transducerarray may be the smallest discrete component of an array. For example,in the case of an array of piezoelectric transducer elements, eachelement may be a single piezoelectric crystal.

As used herein, the terms “transmit element” and “receive element” maycarry their ordinary meanings as understood by those skilled in the artof ultrasound imaging technologies. The term “transmit element” mayrefer without limitation to an ultrasound transducer element which atleast momentarily performs a transmit function in which an electricalsignal is converted into an ultrasound signal. Similarly, the term“receive element” may refer without limitation to an ultrasoundtransducer element which at least momentarily performs a receivefunction in which an ultrasound signal impinging on the element isconverted into an electrical signal. Transmission of ultrasound into amedium may also be referred to herein as “insonifying.” An object orstructure which reflects ultrasound waves may be referred to as a“reflector” or a “scatterer.”

As used herein the term “aperture” refers without limitation to one ormore ultrasound transducer elements collectively performing a commonfunction at a given instant of time. For example, in some embodiments,the term aperture may refer to a group of transducer elements performinga transmit function. In alternative embodiments, the term aperture mayrefer to a plurality of transducer elements performing a receivefunction. In some embodiments, group of transducer elements forming anaperture may be redefined at different points in time. FIG. 3demonstrates multiple apertures used in a multiple aperture ultrasoundprobe. An aperture of the probe has up to three distinct features.First, it is often physically separated from other transducers locatedin other apertures. In FIG. 3, a distance ‘d’ physically separatesaperture 302 from aperture 304. Distance ‘d’ can be the minimum distancebetween transducer elements on aperture 302 and transducer elements onaperture 304. In some embodiments, no transducer elements are disposedalong the distance ‘d’ between the physical separation of apertures 302and 304. In some embodiments, the distance can be equal to at leasttwice the minimum wavelength of transmission from the transmit aperture.Second, the transducer elements of an aperture need not be in the samerectangular or horizontal plane. In FIG. 3, all the elements of aperture304 have a different vertical position ‘j’ from any element of aperture302. Third, apertures do not share a common line of sight to the regionof interest. In FIG. 3, aperture 302 has a line of sight ‘a’ for point(i,j), while aperture 304 has a line of sight ‘b’. An aperture mayinclude any number of individual ultrasound elements. Ultrasoundelements defining an aperture are often, but not necessarily adjacent toone another within an array. During operation of a multi-apertureultrasound imaging system, the size of an aperture (e.g. the numberand/or size and/or position of ultrasound elements) may be dynamicallychanged by re-assigning elements.

As used herein the term “point source transmission” may refer to anintroduction of transmitted ultrasound energy into a medium from singlespatial location. This may be accomplished using a single ultrasoundtransducer element or combination of adjacent transducer elementstransmitting together. A single transmission from said element(s)approximates a uniform spherical wave front, or in the case of imaging a2D slice it creates a uniform circular wave front within the 2D slice.This point source transmission differs in its spatial characteristicsfrom a “phased array transmission” which focuses energy in a particulardirection from the transducer element array. Phased array transmissionmanipulates the phase of a group of transducer elements in sequence soas to strengthen or steer an insonifying wave to a specific region ofinterest. A short duration point source transmission is referred toherein as a “point source pulse.” Likewise, a short duration phasedarray transmission is referred to herein as a “phased array pulse.”

As used herein, the terms “receive aperture,” “insonifying aperture,”and/or “transmit aperture” can carry their ordinary meanings asunderstood by those skilled in the art of ultrasound imaging, and mayrefer to an individual element, a group of elements within an array, oreven entire arrays within a common housing, that perform the desiredtransmit or receive function from a desired physical viewpoint oraperture at a given time. In some embodiments, these various aperturesmay be created as physically separate components with dedicatedfunctionality. In alternative embodiments, the functionality may beelectronically designated and changed as needed. In still furtherembodiments, aperture functionality may involve a combination of bothfixed and variable elements.

In some embodiments, an aperture is an array of ultrasound transducerswhich is separated from other transducer arrays. Such multiple apertureultrasound imaging systems provide greatly increased lateral resolution.According to some embodiments, a multi-aperture imaging method comprisesthe steps of insonifying a target object with an ultrasound pulse from afirst aperture, detecting returned echoes with a second aperturepositioned at a distance from the first aperture, determining therelative positions of the second aperture with respect to the firstaperture, and processing returned echo data to combine images whitecorrecting for variations in speed-of-sound through the target object.

In some embodiments, a distance and orientation between adjacentapertures may be fixed relative to one another, such as by use of arigid housing. In alternative embodiments, distances and orientations ofapertures relative to one another may be variable, such as with amovable linkage. In further alternative embodiments, apertures may bedefined as groups of elements on a single large transducer array wherethe groups are separated by at least a specified distance. For example,some embodiments of such a system are shown and described in U.S.Provisional Patent Application No. 61/392,896, filed Oct. 13, 2010,titled “Multiple Aperture Medical Ultrasound Transducers”. In someembodiments of a multi-aperture ultrasound imaging system, a distancebetween adjacent apertures may be at least a width of one transducerelement. In alternative embodiments, a distance between apertures may beas large as possible within the constraints of a particular applicationand probe design.

A multi-aperture ultrasound imaging system with a large effectiveaperture (the total aperture of the several sub apertures) can be madeviable by compensation for the variation of speed-of-sound in the targettissue. This may be accomplished in one of several ways to enable theincreased aperture to be effective rather than destructive, as describedbelow.

FIG. 1A illustrates one embodiment of a simplified multi-apertureultrasound imaging system 100 comprising two apertures, aperture 102 andaperture 104. Each of apertures 102 and 104 can comprise a plurality oftransducer elements. In the two-aperture system shown in FIG. 1A,aperture 102 can comprise transmit elements T1 . . . Tn to be usedentirely for transmit functions, and aperture 104 can comprise receiveelements R1 . . . . Rm to be used entirely for receive functions. Inalternative embodiments, transmit elements may be interspersed withreceive elements, or some elements may be used both for transmit andreceive functions. The multi-aperture ultrasound imaging system 100 ofFIG. 1A can be configured to be placed on a skin surface of a patient toimage target object or internal tissue T with ultrasound energy. Asshown in FIG. 1A, aperture 102 is positioned a distance “a” from tissueT, and aperture 104 is positioned a distance “b” from tissue T. Alsoshown in FIG. 1A, MAUI electronics may be coupled to the transmit andreceive apertures 102 and 104. In some embodiments, the MAUI electronicscan comprise a processor, control system, or computing system, includinghardware and software configured to control the multi-aperture imagingsystem 100. In some embodiments, the MAUI electronics can be configuredto control the system to transmit an omni-directional unfocusedultrasound waveform from an aperture, receive echoes on an aperture, andform images from the transmitted waveform and the received echoes. Aswill be described in further detail below, the MAUI electronics can beconfigured to control and achieve any of the methods described herein.

Ultrasound elements and arrays described herein may also bemulti-function. That is, the designation of transducer elements orarrays as transmitters in one instance does not preclude their immediatere-designation as receivers in the next instance. Moreover, embodimentsof the control system described herein include the capabilities formaking such designations electronically based on user inputs or pre-setscan or resolution criteria.

Another embodiment of a multi-aperture ultrasound imaging system 200 isshown in FIG. 2 and includes transducer elements arranged to form threeapertures 202, 204, and 206. In one embodiment, transmit elements T1 . .. Tn in aperture 202 may be used for transmit, and receive elementsR_(R) 1 . . . R_(R)m in apertures 204 and 206 may be used for receive.In alternative embodiments, elements in all the apertures may be usedfor both transmit and receive. The multi-aperture ultrasound imagingsystem 200 of FIG. 2 can be configured to image tissue T with ultrasoundenergy. Also shown in FIG. 2, MAUI electronics may be coupled to thetransmit and receive apertures 202 and 204. In some embodiments, theMAUI electronics can comprise a processor, control system, or computingsystem, including hardware and software configured to control themulti-aperture imaging system 200. In some embodiments, the MAUIelectronics can be configured to control the system to transmit anomni-directional unfocused ultrasound waveform from an aperture, receiveechoes on an aperture, and form images from the transmitted waveform andthe received echoes. As will be described in further detail below, theMAUI electronics can be configured to control and achieve any of themethods described herein.

Multi-aperture ultrasound imaging systems described herein may beconfigured to utilize transducers of any desired construction. Forexample, 1D, 1.5D, 2D, CMUT or any other transducer arrays may beutilized in multi-aperture configurations to improve overall resolutionand field of view.

Point Source Transmission

In some embodiments, acoustic energy may be transmitted to as wide atwo-dimensional slice as possible by using point source transmission.For example, in some embodiments, a transmit aperture, such as transmitapertures 102 or 202 in FIGS. 1A and 2, respectively, may transmitacoustic energy in the form of a point source pulse from a singlesubstantially omni-directional transducer element in an array. Inalternative embodiments, a plurality of transducer elements may beprovisioned to transmit a point source pulse that is relatively wide inthree dimensions to insonify objects in a three dimensional space. Insuch embodiments, all of the beam formation may be achieved by thesoftware or firmware associated with the transducer arrays acting asreceivers. There are several advantages to using a multi-apertureultrasound imaging technique by transmitting with a point source pulserather than a phased array pulse. For example when using a phased arraypulse, focusing tightly on transmit is problematic because the transmitpulse would have to be focused at a particular depth and would besomewhat out of focus at all other depths. Whereas, with a point sourcetransmission an entire two-dimensional slice or three-dimensional volumecan be insonified with a single point source transmit pulse.

Each echo detected at a receive aperture, such as receive apertures 104or 204/206 in FIGS. 1A and 2, respectively, may be stored separately. Ifthe echoes detected with elements in a receive aperture are storedseparately for every point source pulse from an insonifying or transmitaperture, an entire two-dimensional image can be formed from theinformation received by as few as just one element. Additional copies ofthe image may be formed by additional receive apertures collecting datafrom the same set of insonifying point source pulses. Ultimately,multiple images can be created simultaneously from one or more aperturesand combined to achieve a comprehensive 2D or 3D image.

Although several point source pulses are typically used in order toproduce a high-quality image, fewer point source pulses are requiredthan if each pulse were focused on a particular scan line. Since thenumber of pulses that can be transmitted in a given time is strictlylimited by the speed of ultrasound in tissue, this yields the practicaladvantage that more frames can be produced per second by utilizing apoint source pulse. This is very important when imaging moving organs,and in particular, the heart.

In some embodiments, a spread spectrum waveform may be imposed on atransmit aperture made up of one or more ultrasound transducer elements.A spread spectrum waveform may be a sequence of frequencies such as achirp (e.g., frequencies progressing from low to high, or vice versa),random frequency sequence (also referred to as frequency hop), or asignal generated by a pseudo random waveform (PN sequence). Thesetechniques can be collectively referred to as pulse compression. Pulsecompression provides longer pulses for greater depth penetration withouttoss of depth resolution. In fact, the depth resolution may be greatlyimproved in the process. Spread spectrum processing typically involvesmuch more signal processing in the form of matched filtering of each ofthe received signals before the delay and summation steps. The aboveexamples of transmit pulse forms are provided for illustration only. Thetechniques taught herein may apply regardless of the form of thetransmit pulse.

Basic Image Rendering

FIG. 1A illustrates one embodiment of a multi-aperture ultrasoundimaging system 100 containing a first aperture 102 with ultrasoundtransmitting elements T1, T2, . . . Tn and a second aperture 104 withultrasound receive elements R1, R2, . . . Rm. This multi-apertureultrasound imaging system 100 is configured to be placed on the surfaceof an object or body to be examined (such as a human body). In someembodiments, both apertures may be sensitive to the same plane of scan.In other embodiments, one of the apertures may be in a different planeof scan. The mechanical and acoustic position of each transducer elementof each aperture must be known precisely relative to a common referencepoint or to each other.

In one embodiment, an ultrasound image may be produced by insonifyingthe entire region to be imaged, such as internal tissue or target objectT, (e.g., a plane through the heart, organ, tumor, or other portion ofthe body) with a transmitting element (e.g., transmit element T1 ofaperture 102), and then receiving echoes from the entire imaged plane ona receive element (e.g. receive element R1 of aperture 104). In someembodiments, receive functions may be performed by all elements in thereceive probe (e.g., R1 through Rm). In alternative embodiments, echoesare received on only one or a select few elements of the receiveaperture. The method proceeds by using each of the elements on thetransmitting aperture 102 (e.g., T2, . . . Tn) and insonifying theentire region to be imaged with each of the transmitting elements inturn, and receiving echoes on the receive aperture after eachinsonifying pulse. Transmit elements may be operated in any desiredsequential order, and need not follow a prescribed pattern.Individually, the images obtained after insonification by eachtransmitting element may not be sufficient to provide a high resolutionimage, but the combination of all the images may provide a highresolution image of the entire region to be imaged. For a scanning pointrepresented by coordinates (i,j) as shown in FIG. 1A, it is a simplematter to calculate the total distance “a” from a particular transmitelement Tx to an element of internal tissue or target object at (i,j),and the distance “b” from that point to a particular receive element.These calculations may be performed using basic trigonometry. The sum ofthese distances is the total distance traveled by one ultrasound wave.

When the speed of ultrasound in tissue is assumed to be uniformthroughout the tissue, it is possible to calculate the time delay fromthe onset of the transmit pulse to the time that an echo is received atthe receive element. (Non uniform speed-of-sound in tissue is discussedbelow.) This one fact means that a scatterer (i.e., a reflective pointwithin the target object) is a point in the medium for which a+b=thegiven time delay. The same method can be used to calculate delays forany point in the desired tissue to be imaged, creating a locus ofpoints. FIG. 1B demonstrates that points (g,h), (i,j), (k,m), (n,p)(q,r), (s,t) all have the same time delay for transmit element T₁ andreceive element R₁. A map of scatter positions and amplitudes can berendered by tracing the echo amplitude to all of the points for thelocus of equal-time-delay points. This locus takes the form of anellipse 180 with foci at the transmit and receive elements. FIG. 1B alsoillustrates MAUI electronics, which can comprise the MAUI electronicsdescribed above with reference to FIGS. 1A and 2.

The fact that all points on the ellipse 180 are returned with the sametime delay presents a display challenge, since distinguishing pointsalong the ellipse from one another within a single image is notpossible. However, by combining images obtained from multiple receivepoints, the points may be more easily distinguished, since theequal-time-delay ellipses defined by the multiple receive apertures willbe slightly different.

FIG. 1C shows that with a transmit pulse from element T1, echoes from asingle scatterer (n,p) are received by different receive elements suchas R1, R2, and R3 at different times. The loci of the same scatterer canbe represented by ellipses 180, 185 and 190 of FIG. 1C. The location atwhich these ellipses intersect (point n,p) represents the true locationof the scatterer. Beam forming hardware, firmware, or software cancombine the echoes from each receive element to generate an image,effectively reinforcing the image at the intersection of the ellipses.In some embodiments, many more receiver elements than the three shownmay be used in order to obtain a desirable signal-to-noise ratio for theimage. FIG. 1C also illustrates MAUI electronics, which can comprise theMAUI electronics described above with reference to FIGS. 1A and 2.

A method of rendering the location of all of the scatterers in thetarget object, and thus forming a two dimensional cross section of thetarget object, will now be described with reference to multi-apertureultrasound imaging system 300 of FIG. 3. FIG. 3 illustrates a grid ofpoints to be imaged by apertures 302 and 304. A point on the grid isgiven the rectangular coordinates (i,j). The complete image will be atwo dimensional array called “echo.” In the grid of FIG. 3, mh is themaximum horizontal dimension of the array and mv is the maximum verticaldimension. FIG. 3 also illustrates MAUI electronics, which can comprisethe MAUI electronics described above with reference to FIGS. 1A and 2.

In one embodiment, the following pseudo code may be used to accumulateall of the information to be gathered from a transmit pulse from onetransmit element (e.g., one element of T1 . . . Tn from aperture 302),and the consequent echoes received by one receive element (e.g., oneelement of R1 . . . Rm from aperture 304) in the arrangement of FIG. 3.

for (i = 0; i < mh; i++){ for (j = 0;j < mv; j++){ compute distance acompute distance b compute time equivalent of a+b echo[ i ][ j ] =echo[i ][ j]+stored received echo at the computed time delay. } }

The fixed delay is primarily the time from the transmit pulse until thefirst echoes are received. As will be discussed later, an increment canbe added or subtracted to compensate for varying fat layers.

A complete two dimensional image may be formed by repeating this processfor every receive element in aperture 304 (e.g., R1 . . . Rm). In someembodiments, it is possible to implement this code in parallel hardwareresulting in real time image formation.

Combining similar images resulting from pulses from other transmitelements will improve the quality (e.g., in terms of signal-to-noiseratio) of the image. In some embodiments, the combination of images maybe performed by a simple summation of the single point source pulseimages (e.g., coherent addition). Alternatively, the combination mayinvolve taking the absolute value of each element of the single pointsource pulse images first before summation (e.g., incoherent addition).In some embodiments, the first technique (coherent addition) may be bestused for improving lateral resolution, and the second technique(incoherent addition) may be best applied for the reduction of specklenoise. In addition, the incoherent technique may be used with lessprecision required in the measurement of the relative positions of thetransmit and receive apertures. A combination of both techniques may beused to provide an optimum balance of improved lateral resolution andreduced speckle noise. Finally, in the case of coherent addition, thefinal sum should be replaced by the absolute value of each element, andin both cases, some form of compression of the dynamic range may be usedso that both prominent features and more-subtle features appear on thesame display. In some embodiments, additional pixel locations arelocated on a grid without scan-conversion.

In some embodiments, compression schemes may include taking thelogarithm (e.g., 20 log₁₀ or “dB”) of each element before display, ortaking the nth root (e.g., 4^(th) root) of each element before display.Other compression schemes may also be employed.

Referring still to FIG. 3, any number of receive probes and transmitprobes may be combined to enhance the image of scatterer (i,j) as longas the relative positions of the transducer elements are known to adesigned degree of precision, and all of the elements are in the samescan plane and are focused to either transmit energy into the scan planeor receive energy propagated in the scan plane. Any element in any probemay be used for either transmit or receive or both.

The speed-of-sound in various soft tissues throughout the body can varyby +/−10%. Using typical ultrasound techniques, it is commonly assumedthat the speed-of-sound is constant in the path between the transducerand the organ of interest. This assumption is valid for narrowtransducer arrays in systems using one transducer array for bothtransmit and receive. However, the constant speed-of-sound assumptionbreaks down as the transducer's aperture becomes wider because theultrasound pulses pass through more tissue and possibly diverse types oftissue, such as fat, muscle, blood vessels, etc. Tissue diversity underthe width of the transducer array affects both the transmit and thereceive functions.

When a scatterer is insonified by a point source pulse from a singletransmit element, it reflects back an echo to all of the elements of thereceiver group. Coherent addition of images collected by elements inthis receive aperture can be effective if the speed-of-sound variationsin the paths from scatterer (i,j) to each of the receiver elements donot exceed +−180 degrees phase shift relative to one path chosen asreference. Referring to FIG. 3, the maximum size of the receive aperturefor which coherent addition can be effective is dependent on tissuevariation within the patient and cannot be computed in advance. However,a practical maximum for a particular transmit frequency can bedetermined from experience.

When insonifying with unfocused point source pulses, the aperture sizeof the transmit group is not highly critical since variation in the pathtime from transmitter elements to a scatterer such as scatterer (i,j)will change only the displayed position of the point. For example, avariation resulting in a phase shift of 180 degrees in the receive pathsresults in complete phase cancellation when using coherent addition,whereas the same variation on the transmit paths results in a displayedposition error of only a half wavelength (typically about 0.2 mm), adistortion that would not be noticed.

Thus, in a multi-aperture imaging system with one aperture used only fortransmit and the other used only for receive during a singletransmit/receive cycle, as is illustrated in FIG. 1A, very littleadditional compensation for the speed-of-sound variation is needed.Although the aperture has been increased from element T1 to Rm which canbe many times the width of a conventional sector scanner probe, theconcern of destructive interference of the signals from scatterer (i,j)is independent of the width of the transmit aperture or the separationof the apertures, and is dependent only on the width of the receiveaperture (element R1 to Rm). The standard width for which speed-of-soundvariation presents a minimal problem in practice is about 16-20 mm for3.5 MHz systems (and smaller for higher frequencies). Therefore, noexplicit compensation for speed-of-sound variation is necessary if thereceive aperture has the same or smaller width than standard apertures.

Substantial improvement in lateral resolution is achieved with a receiveaperture of the same width as a conventional single array 1D, 1.5D or 2Dultrasound probe used for both transmit and receive, because receivedenergy when imaging adjacent cells (i.e., regions of the target object)to that which represents a scatterer is dependent on the time differencebetween when an echo is expected to arrive and the time that it actuallyarrives. When the transmit pulse originates from the same array used forreceive, the time difference is small. However, when the transmit pulseoriginates from a second array at some distance from the receive array,the time difference is larger and therefore more out of phase with thesignal for the correct cell. The result is that fewer adjacent cellswill have signals sufficiently in phase to falsely represent the truescatterer.

Referring to FIG. 4, consider the signal received at a single element(e.g., one of receive elements R1 . . . Rm) of a receive aperture 404from a scatterer at “S”. If both the transmit and receive functions areperformed on the same element, the time for the ultrasound to propagateto “S” and be returned would be 2a/C (where C is the speed-of-sound intissue). When the reconstruction algorithm is evaluating the signalreceived for a possible scatterer in an adjacent cell “S” separated “c”distance from the true scatterer, “S”, the expected time of arrival is2(sqrt(a²+c²)/C). When “c” is small, this time is almost the same and sothe signal from “S” will be degraded only slightly when estimating themagnitude of the scatterer “S′” in the adjacent cell. FIG. 4 alsoillustrates MAUI electronics, which can comprise the MAUI electronicsdescribed above.

Now consider moving the transmitting aperture 402 away from the receiveaperture 404 by an angle theta (“θ”). For convenience in comparison, letthe distance “b” from aperture 402 to scatterer “S” be equal to thedistance “a” from aperture 404 to scatterer “S”. The time for theultrasound to propagate from the transmit aperture 402 to “S” and bereturned to the receive aperture 404 would still be (a+b)/C=2a/C (witha=b), but the expected time for the signal to propagate to the adjacentcell “S” would be (d+sqrt(a²+c²)/C=(sqrt((a sin θ−c)²+(a cosθ)²)+sqrt(a²+c²))/C. The difference between the expected time of arrivaland actual would then be Diff=(sqrt((a sin θ−c)²+(a cosθ)²)+sqrt(a²+c²)−2a)/C.

To put some numbers in this equation, suppose that the separation ofaperture 402 and aperture 404 is only 5 degrees, distance a=400 cells,and distance c=1 cell. Then the ratio of the difference intime-of-arrival for θ=5 degrees to that for θ=0 degrees is 33.8. Thatis, the drop off of display amplitude to adjacent cells is 33 timesfaster with θ=5 degrees. The larger difference in time-of-arrivalgreatly simplifies the ability to uniquely distinguish echo informationfrom adjacent cells. Therefore, with high theta angles, the display of apoint will be less visible as noise in adjacent cells and the resultwill be higher resolution of the real image. With multiple aperturetransmitters and receivers, we can make the angle as high as needed toimprove resolution.

Simulation for a realistic ultrasound system with multiple reflectors inmultiple cells shows that the effect is still significant, but not asdramatic as above. For a system comprising a receive aperture of 63elements, a θ of 10 degrees, and a transmit pulse from a point-sourcetransmit aperture that extends for 5 cycles with cosine modulation, thelateral spread of the point spread function was improved by a factor of2.3.

Explicit Compensation for Speed-of-Sound Variation

A single image may be formed by coherent averaging of all of the signalsarriving at the receiver elements as a result of a single point sourcepulse for insonification. Summation of these images resulting frommultiple point source pulses can be accomplished either by coherentaddition, incoherent addition, or a combination of coherent addition bygroups and incoherent addition of the images from the groups. Coherentaddition (retaining the phase information before addition) maximizesresolution whereas incoherent addition (using the magnitude of thesignals and not the phase) minimizes the effects of registration errorsand averages out speckle noise. Some combination of the two modes may bepreferred. Coherent addition can be used to average point source pulseimages resulting from transmit elements that are close together andtherefore producing pulses transmitted through very similar tissuelayers. Incoherent addition can then be used where phase cancellationwould be a problem. In the extreme case of transmission time variationdue to speed-of-sound variations, 2D image correlation can be used toalign images prior to addition.

When an ultrasound imaging system in a second aperture, using the secondaperture for receiving as well as transmitting produces much betterresolution. In combining the images from two or more receive arrays; itis possible and beneficial to use explicit compensation for thespeed-of-sound variation.

Consider the tissue layer model for the three-aperture ultrasoundimaging system 500 as shown in FIG. 5, which illustrates the effects ofvarying thicknesses of different types of tissue, such as fat or muscle.A fat layer “F” is shown in FIG. 5, and the thickness of the tissuelayers f1, f2, and f3 under each aperture 502, 504, and 506,respectively, is different and unknown. It is not reasonable to assumethat the tissue layer at aperture 506 will be the same as at aperture504, and so coherent addition of the signals from all of the receiveelements together is not usually possible. In one example, if the tissuelayer at aperture 504 were as much as 3 cm larger than that at aperture506, this corresponds to about 3 wavelengths (at 3.5 MHz) displacementof the signals, but this is only 1.3 mm displacement of therepresentation of the deep tissues. For such small displacements, only atiny amount of geometric distortion of the image would be observed.Therefore, although coherent addition is not possible, incoherentaddition with displacement of one image relative to the other ispossible.

Image comparison techniques may be used to determine the amount ofdisplacement needed to align image frames from left and right apertures(e.g., apertures 506 and 504, respectively). In one embodiment, theimage comparison technique can be cross-correlation. Cross-correlationinvolves evaluating the similarity of images or image sections toidentify areas with a high degree of similarity. Areas with at least athreshold value of similarity may be assumed to be the same. Thus, byidentifying areas within images with high degrees of similarity, oneimage (or a section thereof) may be shifted such that areas withsubstantial similarity overlap and enhance overall image quality. FIG. 5also illustrates MAUI electronics, which can comprise the MAUIelectronics described above.

Further, these image comparison techniques can also be used by applyingsub-image analysis, which can be used to determine displacement ofsub-images and accommodate for localized variation in speed-of-sound inthe underlying tissue. In other words, by breaking down the images intosmaller segments (e.g. halves, thirds, quarters, etc), small portions ofa first image may be compared to the corresponding small portion of asecond image. The two images may then be combined by warping to assurealignment. Warping is a technique understood by those skilled in theart, and is described, for example in U.S. Pat. No. 7,269,299 toSchroeder.

The same technique of incoherent addition of images from multiplereceive transducer arrays may be applied to any number of apertures. Thesame idea may be applied even to a single element array which is toowide to be used for coherent addition all at once. An ultrasound imagingsystem with a single wide array of elements may be divided into sections(apertures) each of which is small enough for coherent addition, andthen the images resulting from these sections may be combinedincoherently (with displacement if necessary).

Even a slight distortion of the image may be compensated for withsufficient computational power. Image renderings may be computed for onereceive array using varying amounts of delay in the rendering algorithm(echo[i][j]=echo[i][j]+stored receive echo at the computed time+delay).Then the best matched of these (by cross-correlation or some othermeasure of acuity) may be incoherently added to the image from the otherreceive array(s). A faster technique includes calculating the crosscorrelation network for the uncorrected pair of images, and feeding thisinto a neural network trained to pick the correction delay.

Because multiple aperture ultrasound systems that can correct for speedof sound incongruences allow for significantly larger apertures, someembodiments of the multi-aperture ultrasound systems described hereincan have apertures located 10 cm apart from one another. Sinceresolution is proportional to 2λ/D, this larger aperture leads to higherresolution of tissues located well below the surface of the skin. Forinstance, the renal arteries are frequently located 10 cm to 15 cm belowthe skin and are 4 mm to 6 mm in size near the abdominal aorta. Phasedarray, linear array and synthetic aperture ultrasound systems usuallycannot detect this physiology in most patients; specifically because theaperture size is not large enough to have adequate lateral resolution.Typically, phased array systems have aperture sizes of approximately 2cm. Increasing the aperture size from larger than 2 cm to approximately10 cm in a multi-aperture ultrasound system can increase the resolutionby up to 5×.

3D Imaging

In some embodiments, three-dimensional information may be obtained bymoving a two-dimensional imaging system and acquiring 2D slices at anumber of positions or angles. From this information and usinginterpolation techniques, a 3D image at any position or angle may bereconstructed. Alternatively, a 2D projection of all of the data in the3D volume may be produced. A third alternative is to use the informationin a direct 3D display.

Because multi-aperture ultrasound imaging systems may result in widerprobe devices, the easiest way to use them to obtain 3D data is to notmove them on the patient's skin but merely rock them so that the 2Dslices span the 3D volume to be imaged. In some embodiments, amechanical rotator mechanism which records position data may be used toassist in the collection the 2D slices. In other embodiments, a freelyoperated ultrasound probe with precision position sensors (such asgyroscopic sensors) located in the head of the probe may be usedinstead. Such an apparatus allows for complete freedom of movement whilecollecting 2D slices. Finally, intravenous and intracavity probes mayalso be manufactured to accommodate wide apertures. Such probes may bemanipulated in similar ways in order to collect 2D slices.

This combination is particularly desirable for 3D cardiac imaging usinga multi-aperture ultrasound imaging system. Most patients have goodacoustic windows in two intercostal spaces next to the sternum. Amulti-aperture imaging system is ideal in this case since theintervening rib would render a flat probe useless, while a probe with atleast two widely spaced apertures can be positioned such that a sendaperture and a receive aperture align with separate intercostal spaces.Once a probe with multiple apertures is in place, it cannot be rotated,but it can be rocked to obtain the 3D information, A multi-apertureprobe may also be used in the same intercostal space but across thesternum.

3D information may also be obtained directly with multi-aperture imagingsystems having apertures that are not all in the same scan plane. Inthis case the elements making up the transmit aperture preferablypropagate spherical waveforms (rather than circular waveforms confinedto one plane of scan). The elements making up the receive apertures maylikewise be sensitive to energy arriving from all directions (ratherthan being sensitive only to ultrasonic energy in a single plane ofscan). The reconstruction pseudo code provided above may then beextended to three dimensions.

As for additional details pertinent to the present invention, materialsand manufacturing techniques may be employed as within the level ofthose with skill in the relevant art. The same may hold true withrespect to method-based aspects of the invention in terms of additionalacts commonly or logically employed. Also, it is contemplated that anyoptional feature of the inventive variations described may be set forthand claimed independently, or in combination with any one or more of thefeatures described herein. Likewise, reference to a singular item,includes the possibility that there are plural of the same itemspresent. More specifically, as used herein and in the appended claims,the singular forms “a,” “and,” “said,” and the include plural referentsunless the context clearly dictates otherwise. It is further noted thatthe claims may be drafted to exclude any optional element. As such, thisstatement is intended to serve as antecedent basis for use of suchexclusive terminology as “solely,” “only” and the like in connectionwith the recitation of claim elements, or use of a “negative”limitation. Unless defined otherwise herein, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. The breadth of the present invention is not to be limited bythe subject specification, but rather only by the plain meaning of theclaim terms employed.

1. A method of constructing an ultrasound image, comprising:transmitting an omni-directional unfocused ultrasound waveformapproximating a first point source within a transmit aperture on a firstarray through a target region; receiving ultrasound echoes from thetarget region with first and second receiving elements disposed on afirst receive aperture on a second array, the first array beingphysically separated from the second array; determining a first time forthe waveform to propagate from the first point source to a first pixellocation in the target region to the first receiving element, anddetermining a second time for the waveform to propagate from the firstpoint source to the first pixel location in the target region to thesecond receiving element; and forming a first ultrasound image of thefirst pixel by combining the echo received by the first receivingelement at the first time with the echo received by the second receivingelement at the second time.
 2. The method of claim 1 further comprisingrepeating the determining and forming steps for additional pixellocations in the target region.
 3. The method of claim 1 wherein theadditional pixel locations are located on a grid withoutscan-conversion.
 4. The method of claim 1 wherein determining the firsttime and the second time comprises assuming a uniform speed of sound. 5.The method of claim 2 further comprising transmitting a secondomni-directional unfocused ultrasound waveform approximating a secondpoint source within the transmit aperture through the target region;receiving ultrasound echoes from the target region with first and secondreceiving elements disposed on the first receive aperture; determining athird time for the second waveform to propagate from the second pointsource to the first pixel location in the target region to the firstreceiving element, and determining a fourth time for the second waveformto propagate from the second point source to the first pixel location inthe target region to the second receiving element; and forming a secondultrasound image of the first pixel by combining the echo received bythe first receiving element at the third time with the echo received bythe second receiving element at the fourth time.
 6. The method of claim5 further comprising combining the first ultrasound image with thesecond ultrasound image.
 7. The method of claim 6 wherein the combiningstep comprises coherent addition.
 8. The method of claim 6 wherein thecombining step comprises incoherent addition.
 9. The method of claim 6wherein the combining step comprises a combination of coherent additionand incoherent addition.
 10. The method of claim 1 further comprising:receiving ultrasound echoes from the target region with third and fourthreceiving elements disposed on a second receive aperture on a thirdarray, the third array being physically separated from the first andsecond arrays; determining a third time for the waveform to propagatefrom the first point source to the first pixel location in the targetregion to the third receiving element, and determining a fourth time forthe waveform to propagate from the first point source to the first pixellocation in the target region to the fourth receiving element; andforming a second ultrasound image of the first pixel by combining, theecho received by the third receiving element at the third time with theecho received by the fourth receiving element at the fourth time. 11.The method of claim 10 further comprising repeating the determining andforming steps for additional pixel locations in the target region. 12.The method of claim 10 wherein the additional pixel locations arelocated on a grid without scan-conversion.
 13. The method of claim 11further comprising transmitting a second omni-directional unfocusedultrasound waveform approximating a second point source within thetransmit aperture through the target region; receiving ultrasound echoesfrom the target region with first and second receiving elements disposedon the first receive aperture and with the third and fourth receivingelements disposed on the second receive aperture; determining a fifthtime for the second waveform to propagate from the second point sourceto the first pixel location in the target region to the first receivingelement, determining a sixth time for the second waveform to propagatefrom the second point source to the first pixel location in the targetregion to the second receiving element, determining a seventh time forthe second waveform to propagate from the second point source to thefirst pixel location in the target region to the third receivingelement, determining an eighth time for the second waveform to propagatefrom the second point source to the first pixel location in the targetregion to the fourth receiving element; and forming a third ultrasoundimage of the first pixel by combining the echo received by the firstreceiving element at the fifth time with the echo received by the secondreceiving element at the sixth time, and forming a fourth ultrasoundimage of the first pixel by combining the echo received by the thirdreceiving element at the seventh time with the echo received by thefourth receiving element at the eighth time.
 14. The method of claim 13further comprising combining the first, second, third, and fourthultrasound images.
 15. The method of claim 14 wherein the combining stepcomprises coherent addition.
 16. The method of claim 14 wherein thecombining step comprises incoherent addition.
 17. The method of claim 14wherein the combining step comprises a combination of coherent additionand incoherent addition.
 18. The method of claim 10 further comprisingcombining the first ultrasound image with the second ultrasound image.19. The method of claim 18 wherein the combining step comprises coherentaddition.
 20. The method of claim 18 wherein the combining stepcomprises incoherent addition.
 21. The method of claim 18 wherein thecombining step comprises a combination of coherent addition andincoherent addition.
 22. The method of claim 13 further comprisingcomparing the first ultrasound image to the second, third, and fourthultrasound images to determine displacements of the second, third, andfourth ultrasound images relative to the first ultrasound image.
 23. Themethod of claim 22 further comprising correcting the displacements ofthe second, third, and fourth ultrasound images relative to the firstultrasound image and then combining the first, second, third and fourthultrasound images.
 24. The method of claim 22 further comprisingadjusting the third, fourth, fifth, sixth, seventh, and eighth times tocorrect the displacements of the second, third, and fourth ultrasoundimages relative to the first ultrasound image.
 25. The method of claim10 further comprising comparing the first ultrasound image to the secondultrasound image to determine a displacement of the second ultrasoundimage relative to the first ultrasound image.
 26. The method of claim 25further comprising correcting the displacement of the second ultrasoundimage relative to the first ultrasound image and then combining thefirst and second ultrasound images.
 27. The method of claim 25 furthercomprising adjusting the third time and the fourth time to correct thedisplacement of the second ultrasound image relative to the firstultrasound image.
 28. The method of claim 1 wherein the first pixel isdisposed outside a plane defined by the point source, the firstreceiving element, and the second receiving element.
 29. The method ofclaim 1 wherein the first pixel is disposed inside a plane defined bythe point source, the first receiving element, and the second receivingelement.
 30. The method of claim 10 wherein the first pixel is disposedoutside a plane defined by the point source, the first receivingelement, and the second receiving element.
 31. The method of claim 10wherein the first pixel is disposed inside a plane defined by the pointsource, the first receiving element, and the second receiving element.32. The method of claim 10 wherein the first pixel is disposed outside aplane defined by the point source, the third receiving element, and thefourth receiving element.
 33. The method of claim 10 wherein the firstpixel is disposed inside a plane defined by the point source, the thirdreceiving element, and the fourth receiving element.
 34. Amulti-aperture ultrasound imaging system, comprising: a transmitaperture on a first array configured to transmit an omni-directionalunfocused ultrasound waveform approximating a first point source througha target region; a first receive aperture on a second array having firstand second receiving elements, the second array being physicallyseparated from the first array, wherein the first and second receivingelements are configured to receive ultrasound echoes from the targetregion; a control system coupled to the transmit aperture and the firstreceive aperture, the control system configured to determine a firsttime for the waveform to propagate from the first point source to afirst pixel location in the target region to the first receivingelement, and is configured to determine a second time for the waveformto propagate from the first point source to the first pixel location inthe target region to the second receiving element, the control systemalso being configured to form a first ultrasound image of the firstpixel by combining the echo received by the first receiving element atthe first time with the echo received by the second receiving element atthe second time.
 35. The multi-aperture ultrasound imaging system ofclaim 34 wherein there are no transducer elements disposed between thephysical separation of the transmit aperture and the first receiveaperture.
 36. The multi-aperture ultrasound imaging system of claim 34wherein the transmit aperture and the first receive aperture areseparated by at least twice a minimum wavelength of transmission fromthe transmit aperture.
 37. The multi-aperture ultrasound imaging systemof claim 34 wherein the transmit aperture and the receive aperturecomprise a total aperture ranging from 2 cm to 10 cm.
 38. Themulti-aperture ultrasound imaging system of claim 34 further comprisinga second receive aperture on a third array having third and fourthreceiving elements, the third array being physically separated from thefirst and second arrays, wherein the third and fourth receiving elementsare configured to receive ultrasound echoes from the target region. 39.The multi-aperture ultrasound imaging system of claim 38 wherein thecontrol system is coupled to the transmit aperture and the first andsecond receive apertures, wherein the control system is configured todetermine a third time for the waveform to propagate from the firstpoint source to a first pixel location in the target region to the thirdreceiving element, and is configured to determine a fourth time for thewaveform to propagate from the first point source to the first pixellocation in the target region to the fourth receiving element, thecontrol system also being configured to form a second ultrasound imageof the first pixel by combining the echo received by the third receivingelement at the third time with the echo received by the fourth receivingelement at the fourth time.
 40. The multi-aperture ultrasound imagingsystem of claim 39 wherein the control system is configured to correct adisplacement of the second ultrasound image relative to the firstultrasound image due to speed of sound variation.
 41. The multi-apertureultrasound imaging system of claim 38 wherein the transmit aperture, thefirst receive aperture, and the second receive aperture are not all in asingle scan plane.